FIG. 4 is a schematic diagram of an exhaust gas treating apparatus of a coal-fired boiler. In a coal boiler 10, a combustion gas 11 generates steam in a generating tube inside a furnace 12, as shown in FIG. 4. The generated steam is subjected to gas-liquid separation in a steam drum 13. The steam is introduced into a superheater 14, and converted into superheated steam which is then used for driving a steam turbine. Thereafter, condensed water flows back to a water tube in the furnace 12 and is revaporized. Furthermore, the combustion gas 11 superheats steam in the superheater 14, then heats water to be fed to the coal boiler 10, in an economizer 15, and is exhausted through an exit of the economizer 15 as an exhaust gas 16. The exhaust gas 16 from the economizer 15 is fed to a denitration unit 17. Thereafter, the exhaust gas 16 heats air 19 by heat exchange in an air heater 18, and then is fed into a dust collector 20. Subsequently, the exhaust gas 16 is fed into a desulfurization unit 21 and is then exhausted into the atmosphere as a purified gas 22.
As the denitration unit 17, a denitration unit has been proposed by which reductive denitration is performed by spraying ammonia (NH3), into the exhaust gas 16 from the boiler 10, on the upstream side of the denitration unit (catalyst unit).
Moreover, to remove mercury contained in the exhaust gas, a system has been proposed in which a chlorinating agent such as HCl is sprayed at the upstream of the denitration unit 17, mercury on the catalyst is oxidized (chlorinated), and the mercury is removed by a wet-type desulfurization unit provided on the downstream side (Patent Document 1).
Patent Document 1: Japanese Patent Application Publication No. 10-230137